WW2 Soviet Heavy Tank Prototypes

Object 701 Prototype No.0

Soviet Union (1944)
Heavy Tank – 1 Prototype Built

The development of a completely new tank meant to replace the IS-2 began in summer 1943 at the ChKZ plant in absolute secrecy, with first models being shown to the state in December. After project approval and funding, further work commenced and a first prototype was built in April 1944 under the index Object 701 and prototype number ‘0’. A series of other prototypes were built thereafter until 1945, when the IS-3 had entered service and the tank was deemed unnecessary. A year later, due to issues with the IS-3 tank, the Object 701 program was revived and would enter service under the name IS-4.


In July 1943, the SKB-2 heavy tank design bureau of the ChKZ (Chelyabinsk Kirov Plant) began the development of a new heavy tank out of its own initiative and funds. This was a few months prior to the IS-2 entering service, but even then, there were several known shortcomings of the tank, such as the stepped driver’s hatch, fuel tank position, and overall armor thickness.

Besides designers from SKB-2, engineers from the Bauman Moscow Technical University and Central Artillery Design Bureau (TsAKB) were called to work on the integration of the transmission and armament, respectively. Chief designer was the head of the bureau, N.L. Dukhov, while the chief engineer was C.N. Makhonin and the senior engineer was L.S. Troyanov, later replaced by M.F. Balzhi. The reasoning behind this project’s creation was that the military needed a well-armored heavy tank, immune to the German 75 mm and 88 mm guns found on the Panther and Tiger tanks.

The GABTU (Main Directorate of Armored Forces) would eventually recognize the need for a better armored heavy tank after deep analysis of the Battle of Kursk and combat performance of the IS-2. Thus, in November 1943, the requirements for a new heavy tank were laid down. It was to have a mass of 55 tonnes, frontal armor thickness ranging between 160 and 200 mm, 800 to 1,000 hp engine, main armament with caliber of either 122 mm or 152 mm, top speed of 35 km/h, and crew of 5 men.

By December, the first draft of the project was ready. A scale model was presented under the name K or K-1. Factory director I.M. Saltzman signed and approved the project on 10 December, and on the same day, the People’s Commissar of Tank Industry, V. A. Malyshev, approved the project and supplied funding from the state.

First draft model of the Object 701, named K or K-1.
Source: Domestic Armored Vehicles 1941-1945

In the following months, the project advanced steadily, and by spring 1944, a second mock-up was built, simply called Object 701. Design and development was carried out by I.M. Zaltsman as director of the project, S.N. Makhonin was chief engineer, N.L. Dukhov as chief designer, and lead designer as L.S. Troyanov. After a commission created by the GABTU evaluated the project, a go-ahead was given for producing two prototypes of the Object 701 tank in as little time as possible. Compared to the previous K-1 model, the new mock-up featured an altered turret, with a better integrated gun mantlet, lower commander cupola, and dropping of the rear-facing machine gun.

Scale model of the Object 701, spring 1944. From this angle, several differences between it and the K-1 can be seen.
Source: Domestic Armored Vehicles 1941-1945
Front view of the Object 701 model.
Source: Domestic Armored Vehicles 1941-1945
Rear view of the Object 701 scale model, showing the stepped rear engine deck.
Source: Domestic Armored Vehicles 1941-1945

A state resolution would confirm the GABTU’s will, and on 8 April 1944, Stalin signed decree No. 5583 giving the go-ahead for production of two prototypes, as well as one or two hulls for ballistic trials. The first of the Object 701 pilot tanks was given the index No.0. It was armed with an 122 mm S-34-II gun and equipped with the 750 hp V-12 engine. Its production began just two days later, on 10 April, and in two weeks, it was ready for factory trials.


The No.0 prototype was ready by late April and underwent factory trials between 10 and 24 April, during which it had traveled 1,230 km. In early June, to better test the reliability and functionality of the prototypes, the turret from prototype No.1 was installed on the hull of prototype No.0 and vice-versa. Trials of prototype No.0 ended on 15 June, 1944. Between 6 August to 3 October, ballistic trials of the S-34-II 122 mm on the prototype No.0 were held, passing the tests after firing 780 rounds.


Despite being created with the intention of replacing the IS-2 as the new generation of Soviet heavy tank, the Object 701 had a vastly different upper hull, but maintained many of the same elements of the IS-2 chassis. The hull had been elongated, with seven roadwheels per side, completely redesigned upper hull, as well as new powerplant and armament.

The front of the hull consisted of a sloped upper frontal plate, splitting into a highly angled cheek plate on each side. A large bulge dominated the upper part of the frontal plate, which gave room for the driver’s head, as well as his direct vision port. Two additional periscopes were mounted on top of his service hatch, right underneath the gun. On the lower part of the frontal plate, two tow hooks were welded. The lower frontal plate was slightly angled inwards and had four spare track links attached. On the right cheek plate, the headlight and horn were mounted. The upper sides of the hull were slightly angled inwards and were covered by two external fuel tanks and a toolbox on each side.

Many chassis components were borrowed from the IS-2, including the roadwheels and idler, which were still rimmed and sprung by torsion bars. In contrast, the sprocket and return rollers were designed specifically for the Object 701.

Due to the lengthening of the hull, the rear of the engine compartment was redesigned compared to the IS-2. Instead of the large slope, a ‘step’ was added, separating the engine compartment from the final drive. A tow cable was mounted here, looping across the final drive deck. On top of the engine deck, four armored grilles protected the engine cooling fans, engine access hatch and three caps for internal fuel tanks. An additional hatch was provided for accessing the fuel filters, but was bolted down. The roof over the final drive and transmission was hinged for easier operation.

The turret maintained the rounded shape and cast construction of the IS-2, but presented new design elements. The front was no longer an elongated sphere, but had more defined cheeks, offering better frontal protection. The gun mantlet was tapered outwards, around the gun. A large bulge was cast in the right turret cheek to give room for the gun controls and aiming devices. Several handles were welded across the turret for riding infantry to hold on to.

The roof of the turret was separated into four plates, of which the center was bolted down to allow for the gun to be removed with relative ease. Two service hatches were provided, for the gunner and commander on the right side, and for the loader on the left.

Front view of the Object 701 No.0 during trials. Note the bulge on the right side of the turret cheek.
Source: Soviet Heavy Tank IS-4
Side view of the first Object 701 prototype, likely before trials, summer 1944.
Source: Soviet Heavy Tank IS-4


The driver was seated in the hull and had two MK-4 fixed periscopes in the hatch on the hull roof, as well as a direct vision slit with armored glass. Besides his driving levers and pedals, he had a gauge panel, two compressed air cylinders for starting the engine and four 6STE-128 batteries. Behind him, in the floor of the hull, was an emergency escape hatch.

In the turret, the gunner was seated on the right side of the gun. For vision, the gunner was equipped with two MK-4 fixed periscopes and a rotating periscope. The turret could be traversed by the gunner using both manual and electric systems, with the rotation speed varying between 0.006 and 7.5º/sec when aiming, and 15º/sec when turning the turret at full speed. The gunner’s main sight was a TSh-17 telescopic sight, fixed to the gun. For vertical traverse, a distinctive cutout in the turret cheek armor was made. The electric trigger was placed directly on the vertical traverse control wheel.

The commander was seated behind the gunner and would operate the 10-RK radio station. For communication, the crew had a TPU-4 BIS-F intercom system. Right above him was the service hatch for him and the gunner.

The loader, situated on the left side of the gun, stood on the rotating turret basket floor. He also had a service hatch in the turret roof with a TK-4 periscope.

For personal defense, the crew had two PPSh submachine guns and 25 F-1 hand-grenades.

Cutout rear view of the first two Object 701 prototypes showing the fighting compartment, ammunition layout and armor thickness.
Source: Domestic Armored Vehicles 1945-1965


The main armament consisted of the 122 mm S-34-II gun, developed at TsAKB during the same period as the Object 701, with similar ballistics to the 122 mm A-19 gun. Compared to the D-25T gun used on the IS-2, it had a better structural design and moreover lacked a muzzle brake, which decreased the horizontal blast, improving safety for nearby infantry. A bore fume evacuation system was also added, ejecting fumes when the rammer was opened, decreasing fumes in the fighting compartment and improving rate of fire. The horizontal gun sliding breech lock was semi-automatic. Another major change was the shift of the gunner’s position to the right of the tank, as opposed to the left, as most common on Soviet tanks of the era. The gun had an elevation of +19° and depression of -3°. The first prototype of the S-34-II gun was mounted on the Object 701 prototype No.0 and was tested on it.

The tank also featured a coaxial 7.62 mm DT machine gun, mounted on the left side of the gun, which could be fired by the gunner, but loaded by the loader, with 26 spare ammunition drums.

The two-part ammunition was scattered in the tank, with the projectiles stowed in the rear turret basket, and cartridges stowed in the hull sides, turret basket and left turret side wall.


Hull armor was 42SM armor steel of medium hardness, with the upper frontal plate being 120 mm thick and angled at 61° from vertical. The lower plate was 150 mm angled at 20° from vertical. Side armor was 160 mm thick, angled at 21° on the upper hull and flat on the lower hull. Rear armor was 120 mm thick. Deck and belly armor were 30 mm.

Turret armor was of cast type 66 of medium hardness. Front and rear varied between 160 to 200 mm and the sides were 170 mm, angled at 20°. The roof plates were 30 to 40 mm thick.


The engine compartment, situated in the rear of the hull, was separated from the central fighting compartment by a firewall. The engine was a V-12 diesel engine, outputting 750 hp and max rpm of 2,100 and developed in the autumn of 1943. It was based on the V-12IS engine and boosted using a supercharger from an AM-38 aviation engine. The original maximal rpm of the engine was 2,200, but this had to be decreased for increased reliability once mounted in the tank.

The planetary transmission, designed by the Bauman Moscow Technical School, featured six forward and 3 reverse gears with a 3-stage gearbox.

On the left side of the engine was the oil tank and a fuel tank, and on the right side, the other two fuel tanks. The four radiators and cooling fans were situated above the fuel and oil tanks and were isolated from the rest of the compartment to prevent dust from entering. This layout was largely borrowed from that used on the German Tiger and Panther tanks, although the components themselves were purposely built for the Object 701.

The V-12 engine was used on the Object 701 prototypes and even production IS-4.
Source: Domestic Armored Vehicles 1945-1965

Following Prototypes

The following prototypes, No.1 and No.2, were produced between May and June 1944, and underwent factory trials in Chelyabinsk supervised by a GABTU commission starting on 27 June (for prototype No.1) and 8 July (for prototype No.2) and were finished by the end of July. The prototypes had small variations, primarily regarding the main armament. Prototype No.1 had a 122 mm S-34-II gun as well as a 50 mm S-11 mortar in the turret, while prototype No.2 had a 100 mm S-34-I gun. After the trials, the prototypes were dismantled.

Object 701 prototype No.1, summer 1944.
Source: Soviet Heavy Tank IS-4

In autumn and winter 1944, prototypes No.3, No.4, and No.5 were built and tested. Prototype No.6 was built and tested in 1945, however by this point, the IS-3 had entered service and interest in the Object 701 disappeared. However, in spring 1946, due to the problems and shortcomings found on the IS-3, the Object 701 was revived, and Prototype No.6 entered service under the name IS-4.

One of the Object 701 prototype No.6 in 1945, equipped with the D-25T. The tank would enter production a year later with minor alterations, under the name IS-4.
Source: Soviet Heavy Tank IS-4


The first Object 701 prototype, while just a proof-of-concept, kickstarted a chain reaction of several prototypes for the project, designed to test different aspects and features, from different armaments to new armor thicknesses. Due to the rapid development and production of new and improved pilot models, prototype No.0 was rapidly forgotten and scrapped. Despite the lengthy development period, the final product in the form of the IS-4 proved to be an obsolete vehicle, designed for a war which had ended several years prior.

Object 701 Prototype No.0. Illustrated by Pavel “Carpaticus” Alexe.

Object 701 Prototype No.0 Specifications

Dimensions (L-W-H) 6,600 – 3,260 – 2,450 mm
Total weight, battle-ready 55-56 tonnes
Crew 4 (Commander, gunner, loader, driver)
Propulsion 750 hp V-12 diesel at 2,100 rpm
Speed 35 – 40 km/h (hypothetical)
Armament 122 mm S-34-II
7.62 mm DT machine gun (coaxial)
Armor UFP: 120 mm
LFP: 150 mm
Side hull: 160 mm
Rear hull; 120 mm
Deck & belly: 30 mm
Turret: 150 to 170 mm
No. Built 1


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